SegWit
The 2017 upgrade that fixed transaction tampering and quietly raised block capacity.
In plain words
SegWit — segregated witness — moved signatures out of the part of a transaction that determines its ID. That fixed malleability: before, a relayer could change a transaction's ID without breaking its signatures — poison for anything built on top, like payment channels. Moving witness data also made room: blocks stretch to as much as four times the old measure. It shipped as a soft fork in August 2017, after two years of war over how Bitcoin should scale.
Why it matters
- Malleability's fix is what made Lightning practical.
- More capacity without a hard fork — no one was forced off the network.
- Proof Bitcoin can upgrade under fire: activated by users and miners, not decree.
Numbers that matter
- August 24, 2017 — activation, at block 481,824.
- 4 million weight units — the block limit SegWit introduced.
- bc1q — the address prefix that came with it.
Go deeper
- BIP 141 — the SegWit specification.
- Taproot
- The blocksize war
Updated 2026-07-30